Liquid crystals are widely used for electronic displays. In these display systems, a liquid crystal cell is typically situated between a polarizer and an analyzer. Incident light polarized by the polarizer passes through a liquid crystal cell and is affected by the molecular orientation of the liquid crystal, which can be altered by the application of a voltage across the cell. The altered light goes into the analyzer. By employing this principle, the transmission of light from an external source, including ambient light, can be controlled.
Contrast, color reproduction, and stable gray scale intensities are important quality attributes for electronic displays which employ liquid crystal technology. The primary factor limiting the contrast of a liquid crystal display (LCD) is the propensity for light to “leak” through liquid crystal elements or cells, which are in the dark or “black” pixel state. The contrast of an LCD is also dependent on the angle from which the display screen is viewed. One of the common methods to improve the viewing angle characteristic of LCDs is to use compensation films. Birefringence dispersion is an essential property in many optical components such as compensation films used to improve the liquid crystal display image quality. Even with a compensation film, the dark state can have an undesirable color tint such as red or blue, if the birefringence dispersion of the compensation film is not optimized.
A material that displays at least two different indices of refraction is said to be birefringent. In general, birefringent media are characterized by three indices of refraction, nx, ny, and nz. The out-of-plane birefringence is usually defined by Δnth=nz−(nx+ny)/2, where nx, ny, and nz are indices in the x, y, and z direction, respectively. Indices of refraction are functions of wavelength (λ). Accordingly, out-of-plane birefringence, given by Δnth=nz−(nx+ny)/2 also depends on λ. Such a dependence of birefringence on λ is typically called birefringence dispersion. The in-plane birefringence is usually defined by Δnin=nx−ny, where nx and ny are indices in the x and y direction, respectively. Indices of refraction are functions of wavelength (λ). Accordingly, in-plane birefringence, given by Δnin=nx−ny also depends on λ. Out-of-plane retardation is related to birefringence by Rth=Δnth×d, where d is the thickness of the optical film and in plane retardation Rin=Δnin×d.
In several generally used LCD modes, the LCD display suffers deterioration in contrast when the displays are viewed from oblique angles due to the birefringence of liquid crystal and the crossed polarizers. Therefore, optical compensating is needed and retardance film with optimized in-plane and out-of plane retardation is needed. The use of biaxial films has been suggested to compensate the OCB (U.S. Pat. No. 6,108,058) and VA (JP1999-95208) LCDs.
Birefringence dispersion is an essential property in many optical components such as compensation films used to improve the liquid crystal display image quality. Adjusting Δnth dispersion, along with in-plane birefringence Δnin dispersion, is critical for optimizing the performance of optical components such as compensation films. In most cases, films made by casting polymer have out-of-plane birefringence. Films made by stretching have in-plane birefringence. For simplicity, Δnth will be considered hereinafter. The Δnth can be negative (101) or positive (100) throughout the wavelength of interest, as shown in FIG. 1. In most cases, film made by casting polymer having a positive intrinsic birefringence, Δnint, gives negative Δnth. Its dispersion is such that the Δnth value becomes less negative at longer wavelength (101). On the other hand, by casting polymer with negative Δnint, one obtains a positive Δnth value with less positive Δnth value at longer wavelength (100). The dispersion behavior, in which the absolute value of Δnth decreases with increasing wavelength, is called “normal” dispersion.
In contrast to normal dispersion, it is often desirable to have the absolute value of Δnth increase with increasing wavelength, which is called “reverse” dispersion (reverse dispersion curves 102 and 103 in FIG. 1). Hereinafter, dispersion constant is defined asD=Δn(450 nm)/Δn(590 nm)
Thus, the optical component has a reverse dispersion whenD<1
These cases of different behaviors in Δnth in principle can be achieved by suitable combination of two or more layers having difference dispersion in Δnth. Such an approach, however, is difficult, as one has to carefully adjust the thickness of each layer. Also, extra process steps are added to manufacturing.
U.S. Pat. No. 6,565,974 discloses controlling birefringence dispersion by means of balancing the optical anisotropy of the main chain and side chain chromophore group of a polycarbonate. Both chromophores in the main chain and side chain have normal dispersion but are arranged in a perpendicular orientation and thus have different signs of birefringence, a positive dispersive segment 200 and a negative dispersive segment 201. The combination of them can be finely tuned. This method enables the generation of a polymer having smaller birefringence (or equivalent retardation value) at shorter wavelength, a reverse dispersion copolymer (203) according to the schematics of FIG. 2. However, the incorporation of two balancing chromophores makes the final material less birefringent. Thus, thick films are needed to achieve adequate retardation.